package: return from shellHook correctly

If our shellHook is being run from a nested nix-shell (see 7a12bc200¹),
then (I think) it is run from a bash function due to the nesting, then
`return` is correct. If its `eval`'d though, then there isn't really a
correct way to early exit. So we can just unconditionally be executed in
a function.

Basically, we have IIFE at home.

[1]: 7a12bc2007

Change-Id: Iacad25cbbf66cde2911604e6061e56ad6212af7e

package.nix

@@ -441,6 +441,7 @@ stdenv.mkDerivation (finalAttrs: {
 
           shellHook = ''
             # don't re-run the hook in (other) nested nix-shells
+            function lixShellHook() {
               if [[ $name != lix-shell-env ]]; then
                 return;
               fi
@@ -467,6 +468,9 @@ stdenv.mkDerivation (finalAttrs: {
                 chmod u+x "$gitcommondir/hooks/commit-msg"
               fi
               unset gitcommondir
+            }
+
+            lixShellHook
           '';
         }
       );